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- Volume 34, Issue 3, 1986
Geophysical Prospecting - Volume 34, Issue 3, 1986
Volume 34, Issue 3, 1986
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INVERSION OF INDUCTION LOGGING MEASUREMENTS*
By A.G. RAMMABSTRACTModel problems of induction logging are considered. Some new inversion algorithms are given.
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WHY WATERGUNS?*
Authors E.L. TREE, R.D. LUGG and J.G. BRUMMITTABSTRACTA single watergun generates a ‘stand‐alone’ narrow pulse of acceptable energy content and broad‐band spectrum.
When multiple units of equal volume are fired simultaneously and are accurately synchronized, carefully depth‐controlled and spaced beyond their near‐field interactive distance, the energy measured in the far‐field increases proportional to the square of the number of individual units employed.
When these multiple units are configured in areal array designs based on distance weighting or varying density distribution of the equal elements, further gains in downward directivity and attenuation of horizontally traveling interference accrue. Such pattern designs take account of the spectral content of the particular watergun used, while the wavenumber response is concentrated to attenuate the dominant interference at water velocity.
Finally, the high‐frequency content, and high repeatability and synchronizing accuracy of the new generation of waterguns improves the high resolution potential to help combat natural earth absorption losses. We should, therefore, keep the ‘in‐line’ dimensions at both source and receiver as short as possible. Unacceptable interference that persists during subsequent processing can be controlled by the long‐array simulation technique.
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A LAYER‐STRIPPING TECHNIQUE FOR THE SUPPRESSION OF WATER‐BOTTOM MULTIPLE REFLECTIONS*
More LessABSTRACTA new method to suppress water‐bottom multiples (water‐bottom reverberations) uses the fact that in the domain of intercept time and ray parameter (τ–p domain) the water‐bottom reverberations are strictly periodical for a horizontal flat sea bottom.
Using this property a comb filter can be designed. The window of the filter should be approximately equal to the duration of a source pulse. The algorithm finds the maximum of the periodical energy throughout the τ–p domain and then designs the comb filter which eliminates the water bottom reverberations from each trace in the τ– p domain. This process can be repeated for higher order reverberations. Finally the τ–p domain with attenuated multiples is transformed back to the conventional x ‐‐ t space.
The method is illustrated on a variety of synthetic data and on a set of real marine CMP data acquired in the North Sea near the Norwegian shore.
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THE SCATTERING OF SH‐CHANNEL WAVES BY A FAULT IN A COAL SEAM*
More LessABSTRACTThe imaging of faults in coal seams by the in‐seam seismic method has now become standard practice. In the UK over 300 surveys have been undertaken and the technique is now part of the exploration arsenal of colliery planners. From these users comes the pressure for two major improvements, namely an increase in range and target identification. This paper is directed towards the latter problem.
It has long been recognized that the reflected channel waves must contain information on the fault structure that caused the reflection, and model experiments have been undertaken to investigate the reflection process. Only recently, however, have attempts been made to quantify the reflection process. Calculations using both the finite‐difference and finite‐element techniques have been carried out, and estimates of the reflection coefficient as a function of frequency have been obtained. The object of this paper is to extend these considerations by calculating analytically the scattering matrix of an SH‐channel wave after interaction at a fault plane. The scattering matrix is calculated as a function of frequency, hade angle, and fault throw.
The method employed is based on the decomposition of the incident SH‐channel wave into Fourier components, the calculation of plane wave reflection and transmission coefficients within the constraints of geometrical acoustics, and finally the synthesis of the scattering matrix by application of the Helmholtz‐Kirchhoff integral. The calculation throughout is restricted to normal modes.
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A COMPARISON OF STATISTICAL AND DETERMINISTIC WIENER DECONVOLUTION OF DEEP‐TOW SEISMIC DATA*
By A. N. BOWENABSTRACTWiener ‘spiking’ deconvolution of seismic traces in the absence of a known source wavelet relies upon the use of digital filters, which are optimum in a least‐squares error sense only if the wavelet to be deconvolved is minimum phase. In the marine environment in particular this condition is frequently violated, since bubble pulse oscillations result in source signatures which deviate significantly from minimum phase. The degree to which the deconvolution is impaired by such violation is generally difficult to assess, since without a measured source signature there is no optimally deconvolved trace with which the spiked trace may be compared.
A recently developed near‐bottom seismic profiler used in conjunction with a surface air gun source produces traces which contain the far‐field source signature as the first arrival. Knowledge of this characteristic wavelet permits the design of two‐sided Wiener spiking and shaping filters which can be used to accurately deconvolve the remainder of the trace. In this paper the performance of such optimum‐lag filters is compared with that of the zero‐lag (one‐sided) operators which can be evaluated from the reflected arrival sequence alone by assuming a minimum phase source wavelet. Results indicate that the use of zero‐lag operators on traces containing non‐minimum phase wavelets introduces significant quantities of noise energy into the seismic record. Signal to noise ratios may however be preserved or even increased during deconvolution by the use of optimum‐lag spiking or shaping filters.
A debubbling technique involving matched filtering of the trace with the source wavelet followed by optimum‐lag Wiener deconvolution did not give a higher quality result than can be obtained simply by the application of a suitably chosen Wiener shaping filter. However, cross correlation of an optimum‐lag spike filtered trace with the known ‘actual output’ of the filter when presented with the source signature is found to enhance signal‐to‐noise ratio whilst maintaining improved resolution.
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THE SCALING OF AIRGUN ARRAYS, INCLUDING DEPTH DEPENDENCE AND INTERACTIONS*
More LessABSTRACTThe article provides a theoretical basis for the extension of the method of scaling law deconvolution to three dimensions using airgun arrays as a sound source.
Earlier papers by the author required the dimensions of the scaled sources to be different while the depths and firing pressures were maintained the same in order to preserve the same dynamics of the scaled sources at scaled time. However, this forces the source ghost to be considered as part of the impulse response of the earth rather than as part of the downgoing source wave. And, in fact, the dynamics of the scaled sources are not the same at the same depth because the ghost reflection modulates the behaviour of the oscillating bubbles generated by the airguns, and this modulation does not scale.
To force the sources to scale properly, including the ghost interaction, the larger source must be put at greater depth, where hydrostatic pressure is greater, and the initial firing pressure must be adjusted accordingly. Thus, the depth, initial firing pressure and gun volume are all variables. The interaction among guns in scaled airgun arrays also scales exactly if the geometry of an array and the depth of its deployment are scaled by the same factor.
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RESISTIVITY SOUNDING ON A HORIZONTALLY STRATIFIED MULTI‐LAYERED EARTH*
Authors S.S. RAGHUWANSHI and BIJENDRA SINGHABSTRACTThe electrical potential due to a point source of current placed on the ground surface is studied for a multi‐layered earth consisting of homogeneous overburden of constant conductivity over a stack of transition layers where conductivity varies with depth according to power and exponential laws in even and odd layers, respectively. The general recursion relations are derived and are used to obtain expressions for the apparent resistivities for Schlumberger and Wenner electrode arrays. Their asymptotic behavior has been studied. The solutions for some particular cases are given: (i) odd layers with conductivities exponentially varying with depth while all even layers (and the first) have constant conductivities; (ii) even layer conductivities varying as a power law while odd number layers are of constant conductivity; and (iii) any two successive layers as transition layers and all others having constant conductivities. Further it is shown that Patella's theory is a particular case of the present study. It is concluded, therefore, that the present treatment is more general as all earlier models consisting of trasition layers can be derived from this study.
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GEOELECTRIC AXIAL DIPOLE SOUNDING CURVES FOR A CLASS OF TWO‐DIMENSIONAL EARTH STRUCTURES*
Authors D. PATELLA and A. TRAMACEREABSTRACTWith the aim of studying the behaviour of geoelectric axial dipole vertical soundings over complex geology, a systematic theoretical approach is presented for a class of earth structures characterized by horizontal and vertical parallel boundary planes. The two‐dimensional cylindrical bodies of infinite length and rectangular cross‐section are constrained to have resistivities satisfying Alfano's condition at every intersection line of the graticule, in order to adopt the image‐point theory. A detailed analysis is performed for models with any number of horizontal boundaries and two vertical discontinuities. The apparent resistivity formulas are obtained and selected apparent resistivity curves are drawn for different parameter combinations and various directions of the sounding expansion axis. The class under consideration contains as a particular case the HVC model elaborated in Alpin's monograph, where only a small collection of master curves is available for the axial array. The reconstruction of those curves by the present formulation shows the existence of large discrepancies. A test based on the transformation to equivalent half‐Schlumberger sounding curves supports the conclusion that an unidentified error must exist in some part of the theoretical approach of the Russian researchers. Finally, some field sounding curves based on geothermal and volcanological surveys are presented and interpreted by complete curve matching, essentially to show the applicability of the theoretical solutions.
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TESTING THE TRANSIEL METHOD IN MINERAL AND GEOTHERMAL EXPLORATION*
More LessABSTRACTTransiel is the name given to a qualitative, time domain induced polarization (IP) method. The aim of the method is twofold: to locate and to distinguish between deep and shallow polarizable media. This discrimination is based on the data analysis, which distinguishes Transiel from the conventional IP methods.
Two case histories are presented: one related to mineral prospecting and the other to geothermal exploration. The information supplied by Transiel on the deep polarizable layers is checked at each test site by an independent inversion of the recorded transients.
At the mineral test site, the method correctly predicts the target location. In this particular survey, the maximum penetration depth of the method is 700 m. At the geothermal test site, a fair correlation is found between IP anomalies and the productive wells. Since the method's penetration depth does not exceed 500 m, we believe that the observed IP anomalies are related to reduction phenomena occurring in the overburden and leading to pyrite formation. We suppose that these phenomena are caused by thermochemical exchanges between the reservoir and the overburden above zones of high reservoir permeability.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)